Find out the path to the folder ../site-packages/ of your corresponding python version. ( For me it was /opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/ for python 2.6 on my Mac with Snoe Leopard.)

Open eclipse preferences and go to PyDev -> Interpreter - Python.

On the left side of the lower box, click on New Folder.

Add the navigate to ../site-packages/ of your corresponding python version.

Hit open.

Hit Apply.

Hit Ok.

And you should be good to go. =)

Thanks @all particionts, who provided hints into the right direction in the comments.